"Alex Ionescu, the ReactOS kernel coordinator, has resigned. Alex first joined the project in 2004, around the 0.2.2 release. Since then, he's been at the center of quite a few squabbles about how to code the kernel. However, Alex has also been responsible for completely rewriting the kernel almost from the ground up. Today, about 60% of the kernel code is probably his. The reason for Alex's departure is because of his joining David Solomon's Expert Seminars as an instructor. Because this job would place him in close contact with many Microsoft programmers and also give him access to other Microsoft properties, continuing with the project would have resulted in possible conflicts of interest."

But Linux was never in danger of being sued out the wazoo for patent infringement. Not by the proprietary Unix vendors, anyway. A certain degree of openness has always been part and parcel of the Unix way.

Heck no. The way things were done in the UNIX world was what made RMS start the free software movement. It was not pretty and open, to say the least.

And yes, the danger is looming over Linux as well, because software patents won't simply go away. They are an abomination by definition and it has reached a point where any piece of software in existence today violates a patent or more.

There are several reasons for which Linux hasn't been taken to court over patents (so far).
1. It would be a big suite, which may well destroy the entire software patent idiocy. Big corps don't want that.
2. Many big corps have invested in Linux. You kick Linux, you kick them. They won't take it lying down, and they have patent portfolios of their own. It would trigger a nuclear patent war from which nobody survives.

But what is to keep Microsoft from suing that hell out of the ReactOS devs, if they ever become a threat?

They might, but it's most likely they would choose other excuses: "reverse engineering" and DMCA is a nice one. Either way, ReactOS does not have the level of endorsement that Linux has, it would have a tougher time fighting Microsoft.

Besides, why bother to sue when you can just hire the main developer and set back the project for months or years?